[sitotis]

I have a DA/SA semi-auto. I pull back the hammer with my thumb to forego the DA pull and put the firearm into an SA-ready condition. If the hammer slips while pulling it back and strikes the firing pin, is there any kind of safety mechanism to keep the gun from firing considering the trigger was not touched?

I have both an H&K P30 and a SIG 226 with which this situation could happen...

[Jim Watson]

If you stay off the trigger, the internal safeties should keep the gun from firing if you let the hammer slip. Should.

[Scimmia]

If there's a half cock notch, that should catch it.

Both of those pistols also have a firing pin block, so even if the half cock notch doesn't catch it, the block would stop the firing pin from going forward enough to hit the primer. You have to have the trigger pulled to move the block out of the way.

Of course, anything mechanical can fail.

[1SOW]

You can also chamber the first round which cocks the hammer for shooting.
If it's for carry, use a mag to chamber the round and cock the hammer, then insert another full mag to top it off..

[sitotis]

These two guns have no manual safety. The reason for my question is that if I have time to cock the hammer and forego the initial DA pull, I was wondering how safe it is to pull back the hammer first with a round already in the chamber.

[David E]

The obvious solution is to learn to master the DA stroke and transition.

If you must cock the hammer for the first shot, use your off-side thumb.

[1SOW]

David E is right on the mark.
A lighter hammer/mainspring can make learning to make the DA to SA transition easier.

[1911Tuner]

sitosis...Yes. It will still fire when you pull the trigger. If a half or quarter cock stops the hammer, you may have to either cock it or pull it through in DA mode...but there's nothing that will disable the gun and keep it from firing.